With advances in telecommunications technology, 2G and 3G networks are expected to co-exist for a long time. To meet the coverage requirement of such different communication networks, more rigorous quality requirements are required for mobile telecommunication systems. In particular, a broadband antenna capable of operating in both the 2G frequency band and the 3G frequency band is strongly desired.
To optimize networks of various communication systems, the antennas used must provide a high precision horizontal beam width. Lobe-shaping should also be taken into account in designing the elevation pattern to suppress the upper side lobe and to realize zero filling of the lower side lobe, thereby attaining more reliable communication quality. Furthermore, polarization diversity technology has been applied to antennas of base stations to eliminate multi-path fading, thus also greatly improving communication quality.
Base station antennas are important outside components of mobile telecommunication systems. Presently, bi-polarization is a major polarization diversity of such base stations. The bi-polarized antennas mainly include those polarized by an angles of ±45°, which mostly include antennas having a horizontal beam width of 65°. The performance of this kind of antenna (with the horizontal beam width of 65°) directly impacts the coverage and polarization diversity gain of the mobile telecommunication systems and therefore impacts working performance of the entire network.
A conventional bi-polarized base station antenna with a polarization angle of ±45° is constructed either of radiation units provided with symmetric dipoles or of microstrip radiation units. The relative operating frequency of this kind of antenna with high cross polarization discrimination is less than 10%, thus influencing the correlation between +45° antenna and −45° antenna and influencing diversity efficiency of the antenna working at a wide frequency range. The value of the cross polarization discrimination also influences the separation between output ports. Further, the gain of the antenna is decreased, the switch time in margin regions is increased, and the communication quality of the network deteriorates due to wide horizontal half power beam width of the symmetric dipole radiation unit. In addition, the working frequency range of a conventional symmetric dipole antenna is only about 13%. Moreover, antennas constructed of microstrip radiation units have an even narrower frequency range of no more than 10%.
A radiation unit is disclosed in U.S. Pat. No. 4,434,425, assigned to GTE Products Corporation and published in 1984, the disclosure of which is incorporated by reference herein. The patent shows a solution to the above issue, in which the high frequency dipole is incorporated into the low frequency dipole, as illustrated in FIG. 1. The combination of the low-frequency antenna radiation unit with the high frequency antenna radiation unit shows a way to realize small-sized, multiple frequency community base station antennas.
A multiple frequency community base station antenna used in mobile communication system is described in U.S. Pat. No. 6,333,720 B1, issued to the German company Kathrein and published in 2001, the disclosure of which is incorporated by reference herein, and is shown in FIG. 2. The apparent interrelationship among the radiation units is the same as that disclosed in U.S. Pat. No. 4,434,425.
However, the radiation units described in both of the above patents suffer from various drawbacks, such as a large frontal projected area and a complicated construction. Additional drawbacks are set forth below.
Firstly, high frequency radiation performance deteriorates due to the coupling effect of the two low frequency dipoles on the high frequency dipole when located between the two low frequency dipoles.
Secondly, if restricted control of the vertical grating lobe of a multiple frequency electronically adjustable base station antenna is required for the communication system, then the pitch between the radiation units is reduced, thus causing more significant coupling between the two low frequency dipoles as well as between the low and high frequency dipoles. In some cases, this coupling is unacceptable and causes great damage to the circuitry and radiation characteristics of the antenna.
Multiple frequency community base station antennas commonly have no high frequency dipole incorporated into the low frequency dipole. By contrast, a low frequency dipole having high frequency dipole included therein has a significantly different impedance performance than a low frequency dipole that does not have such a high frequency dipole contained therein.
Accordingly, the technical evolution of the radiation unit is very complicated, though its design seems simple physically. It is therefore desired to balance the relationship between size and electrical performance, i.e., the technical parameters, of the radiation unit.
It is thus desirable to overcome drawbacks described above and provide a bi-polarized broadband radiation unit of annular type with not only improved performance of various parameters of the radiation unit but also with reduced size thereof.
It is further desirable to provide a linear array antenna with such radiation unit incorporated therein.